The present invention relates to improvements in a noise shaper, an AD converter and a DA converter of the oversampling type using a method of PCM-encoding an audio frequency band signal as stipulated in the specifications of CCITT G.711.
When there is used a method of PCM-encoding an audio frequency band signal as stipulated in the specifications of CCITT G.711, the specifications of CCITT G.714 set forth a stipulation of the transmit-receive separation characteristics of a four-line interface PCM encoding method. In the method of PCM-encoding an audio frequency band signal stipulated in the specifications of CCITT G.711, there are stipulated a compression method, called an A-rule, using approximation with 13 line segments, and a compression method, called a .mu.-rule, using approximation with 15 line segments. For 8-bit PCM data, either compression method requires precision of about 12 bits at the smaller-amplitude side and precision of about 7 bits at the larger-amplitude side.
For realizing an AD converter or a DA converter based on the encoding method above-mentioned, such an AD or DA converter is required to present precision of about 12 bits.
As examples of the AD converter presenting precision of about 12 bits, there have been conventionally developed a variety of AD converters of the sequential comparison type in each of which one analog signal is sequentially compared with each of a plurality of reference levels to obtain a digital signal. This is because the sequential comparison type can be adopted because of the fact that the audio frequency band is relatively narrow, and because such an AD converter is balanced most in view of power consumption and circuit size.
In the AD converter of the sequential comparison type above-mentioned, however, it is required to dispose a sharp pre-filter for limiting the input band. In this connection, such an AD converter is formed with the use of SCF (switched capacitor) technique in order to make the component elements including such a pre-filter in the form of an IC.
With the recent demand for miniaturization in the semiconductor processing technique, there has been developed, as analog data/digital data mutual converting means, an AD converter or DA converter of the oversampling type using a delta-sigma modulating method. In such an oversampling AD converter or DA converter of the delta-sigma modulating type, as shown in FIG. 13, while sampling a signal with frequency Fs which is tens to hundreds times of the upper-limit frequency FBW of the signal band, frequency characteristics are given quantization noise such that the quantization noise presents a peak at frequency of 1/2 of the sampling frequency Fs (Nyquist frequency), that the quantization noise is reduced to a small value in the signal band and that a high-band component of the quantization noise is removed by a digital lowpass filter at a subsequent stage. This method is also called noise shaping.
The oversampling AD converter or DA converter of the delta-sigma modulating type above-mentioned, can effect a highly precise conversion without use of a multiple-bit highly precise DA converting unit used in an AD converter of the sequential conversion type mentioned earlier. Further, the sampling frequency is high with respect to the signal band, so that the specifications required for a pre-filter are advantageously not so severe. With the recent demand for miniaturization in the semiconductor processing technique, there is a tendency to make a digital unit in a compact design with the power consumption lowered. Further, such an AD converting method of the oversampling type has good compatibility with other signal processing LSIs. In this connection, increasing attention will be given to such an AD converting method.
There are instances where, when arranging an AD converter or DA converter adopting the encoding method mentioned earlier, there is formed a converter presenting precision of about 12 bits, by an oversampling AD converter or DA converter of the delta-sigma modulating type. In such a case, it is required to form a second-order delta-sigma modulator in which the oversampling rate is about 100 times.
FIG. 11 shows the arrangement of a noise shaper using a second-order delta-sigma modulator. In FIG. 11, there are shown adders 51, 52, delay devices 53, 54, 55, a quantizer 56 for quantizing an input signal to a +1 or -1 2-level digital signal, and two signal accumulating means 57. 58 formed by the adders 51, 52 and the delay devices 54, 55. A digital output signal Y or 2-level signal is feedbacked to the adders 51, 52 of the signal accumulating means 57, 58 through the delay device 53. The output amplitude (+1, -1) feedbacked through the delay device 53, is set to the maximum amplitude of an input signal X. An output of the signal accumulating means 57 at the former stage is entered into the signal accumulating means 58 at the latter stage, and an output signal of the latter-stage signal accumulating means 58 is entered into the quantizer 56.
The following equation shows system functions of the second-order delta-sigma modulator shown in FIG. 11: EQU Y(z)=X(z)+(1-z.sup.-1).sup.2 .multidot.Q(z) (1)
wherein X is the input, Y is the output and Q is quantization noise.
In the equation (1), Q is quantization noise. This quantization noise Q has been subjected to second-order noise shaping. Accordingly, when the sampling frequency is sufficiently high as compared with the band of the input signal X, the input signal X and the quantization noise Q can be separated from each other by filtering the digital output signal Y.
in the oversampling AD converter or DA converter of the delta-sigma type above-mentioned, the following three methods can be proposed in order to realize, by reducing quantization noise in the signal band, an oversampling AD converter or DA converter of the delta-sigma type which satisfies the transmission characteristics based on the encoding method above-mentioned.
1) To use a higher order delta-sigma modulator PA1 2) To increase the sampling rate PA1 3) To increase the number of the quantizing levels
However, any of the methods 1) to 3) presents the following defects.
More specifically, the transmission characteristics based on the encoding method above-mentioned may present such precision that the signal/noise ratio is about 7 bits when the amplitude of the input signal is great, but are required to present such precision that the signal/noise ratio is about 12 bits when the amplitude of the input signal is small. When, according to any of the quantization noise reducing methods 1) to 3) above-mentioned, there is achieved an arrangement which assures precision of about 12 bits when the amplitude of the input signal is small, there is also assured precision of about 12 bits even when the amplitude of the input signal is great. Accordingly, the specifications of CCITT G.711 are excessively satisfied. This disadvantageously lowers the efficiency.
Further, when a high order delta-sigma modulator is used as mentioned in the above item 1), the quantization noise in the signal band can be sent away toward the higher frequency side, so that the quantization noise in the signal band can be efficiently reduced. However, this disadvantageously requires to increase, with an increase in order, the number of signal accumulating circuits for accumulating each difference signal.
Further, it is theoretically possible to reduce the quantization noise in the signal band by increasing the sampling rate as mentioned in the above item 2). However, such an increase in sampling rate is limited to a certain degree in view of the processing speeds of analog or digital elements and the power consumption.
When there are formed a plurality of quantization levels, instead of one bit, as mentioned in the above item 3), the quantization noise itself can be reduced. However, this disadvantageously requires a great number of comparators for forming the quantizer.